1. Field of the Invention
This invention relates to an improved stripping composition and process for removing ion implanted positive photoresist and other difficult to remove polymeric layers from substrates. More particularly, it relates to such a composition and process which is more aggressive in the removal of ion implanted positive photoresist and similar difficult to remove polymeric layers without causing damage to the underlying substrate. It further relates to a novel prebaking process for making positive photoresist easier to remove with stripping solutions.
2. Description of the Prior Art
Stripping compositions utilizing various amine compounds for the removal of positive photoresist from semiconductor wafers having insulating and/or conductive patterns defined on their surfaces using the photoresist during the manufacture of integrated circuits are known in the art. For example, Posistrip 830, available from EKC Technology, Hayward, Calif. 94540, is the most aggressive commercially available stripper for positive photoresist. The principal active ingredient of Posistrip 830 is 2,(-2 aminoethoxy)ethanol. Another material that is used for this purpose is N-methyl pyrrolidone.
In very large scale integration (VLSI) fabrication, ion implantation is used to add dopant atoms to silicon or other semiconductor material to change the conductivity of the semiconductor material. Examples of such dopants include donor atoms, or n-type dopants, such as phosphorus, arsenic or antimony and acceptor atoms, or p-type dopants, such as boron, aluminum or gallium. The superiority of ion implantation over diffusion doping methods for this purpose has caused it to replace diffusion doping in an increasing number of applications.
To restrict the ionic species from being implanted into unwanted substrate regions, an appropriate masking layer needs to be present on the wafer surface, with openings in the areas where implantation is desired. Many materials are used for such masking purposes in integrated circuit fabrication, including photoresist, SiO.sub.2, Si.sub.3 N.sub.4, polysilicon, metal films, and polyimide.
A thin layer of SiO.sub.2 of, for example, 200-300 Angstroms thickness is usually added in the openings to provide a protective screen against contamination by metals or other impurities during implant. Higher energy implanters are needed, especially for arsenic implants, when this is done as the implanting beam must first penetrate the screening layer in the openings, during which time it loses energy, before arriving at the semiconductor substrate. Modern VLSI processes employ implant doses ranging from about 10.sup.11 to 10.sup.16 ions/cm.sup.2. After heavy doses, a photoresist masking layer may become difficult to remove.
As a result of such removal difficulties, the most common method for removing ion implanted photoresist is an oxygen plasma process, which is time consuming and expensive. Commercially available photoresist stripping solutions, including the Posistrip 830 solution and the N-methyl pyrrolidone mentioned above, have not proven to be suitable for stripping such ion implanted photoresist, especially at heavy dosing levels. The state of the ion implantation processing art is discussed in Stanley Wolf and Richard N. Tauber, Silicon processing for the VLSI Era, Chapter 9, "Ion Implantation for VLSI", pp. 280-330, including the difficulty of stripping ion implanted photoresist.